The present invention relates generally to components used in microwave transmission systems such as phased radar arrays, and more particularly to a novel low loss phase shifter printed on a ferrite substrate that advantageously operates at microwave frequencies and which has ideal characteristics for millimeter wave (MMW) applications.
It is well known that a phase shifter is a key element in phased array radar systems. It is also well known that there are two types of phase shifters used in this application, one being a diode phase shifter and the other being a ferrite phase shifter. Generally, where the application calls for operation at high frequencies (above 10 GHz) and in high power systems, a ferrite phase shifter configuration is utilized.
For instance, such ferrite devices are used to electronically steer the beam of phased array radar systems. A phased array usually consists of thousands of radiating elements, and unless subarray feeding is employed, an array antenna normally requires thousands of phase shifters. Thus, it is highly desirable to utilize low cost phase shifters for array applications.
Conventionally, a ferrite phase shifter must be packaged in a metalized ferrite bar or a ferrite loaded waveguide to support a circularly polarized (CP) wave, which is required to interact with a longitudinal magnetic field. A desired phase shift is achieved by adjusting the bias magnetic field along the axis of the ferrite bar. Problems arise in the fabrication of such devices because the cross section of the ferrite phase shifter is only a fraction of the operating wavelength. The building of such a phase shifter is very difficult and costly because most of which cost is in the machining of the waveguide and the sputtering of a metalized ferrite bar.
Prior art designs also require that a thin quarter-wave plate be inserted in the ferrite bar at the input and output ends in order to convert a linear mode into a circularly polarized mode, and vice versa. For MMW frequencies, it becomes increasingly difficult to make a (square or circular) ferrite bar as small as a pencil lead.
From the above, it should be evident that a ferrite phase shifter with a planar geometry that can utilize printed circuit technology to drastically reduce production costs, and that will advantageously operate at microwave and particularly MMW frequencies, is very desirable.
It should be noted that a ferrite phase shifter with a planar geometry has been developed in the prior art. For example, a microstrip design using a meander line approach was reported in an article entitled "Thin Ferrite Devices for Microwave Integrated Circuits" by Gerard T. Roome, and Hugh A. Hair, in IEEE Transactions on Microwave Theory and Techniques, July 1968, pp. 411-420. This configuration, however, is not very efficient because the circuit can not support a CP wave in a substantial way.
As can be seen in FIG. 8 of this reference, only a small region around the mid point of the quarter-wave segments in the meander line can support a CP wave. To be effective, a configuration that can support a CP wave in a substantial way and maximize its Faraday rotation continuously along the bias magnetic field is needed. This is exactly what the present invention provides.
In contrast to the prior art, the present invention alleviates the problems enumerated above by using three microstrip lines to support a CP wave for maximum interaction with the bias magnetic field through the ferrite substrate. Explicitly, the unique feature of this invention is the effective way to excite the required eigen modes (Right Hand Circularly Polarized [RHCP] and Left Hand Circularly Polarized [LHCP]) in a flat ferrite substrate.